The long-term objectives of this application are to understand how hormones transduce signals to cells. The general strategy is to focus on a superfamily of seven transmembrane-spanning receptors that bind hormones and mediate signals via G proteins. The widespread use of G protein-coupled receptors (GPCRs) as a mechanism of signal transduction into cells make them frequent targets of pharmaceutical drugs;according to one estimate, almost 60% of all drugs act on GPCRs. These include commonly prescribed agents such as beta-blockers (cardiovascular), beta-agonists (asthma), antihistamines (H1- allergies, H2-ulcers), and opiates (pain). An estimated 3% of human genes encode GPCRs;the ligands remain to be identified for nearly half of the receptors, thus offering many new potential drug targets. Despite their biological and medical importance, the molecular mechanisms by which receptors activate G proteins are poorly understood. To address these fundamental questions, this Proposal employs a variety of techniques including genetic screens, computational modeling, and biochemical analyses to study the human complement factor 5 (C5a) receptor, a member of the rhodopsin family of GPCRs. The C5a receptor mediates neutrophil chemotaxis and functions well when expressed in yeast, making possible high-throughput structure/function studies of regions of the receptor that bind hormones or couple to G proteins. Rhodopsin, a photoreceptor activated by light, has been well characterized by biophysical analyses, and is the only GPCR for which a crystal structure is available. The two receptors share similar structures, enabling computational models of the C5a receptor to be generated and for specific models of receptor activation, derived from genetic studies of the C5a receptor, to be tested directly. The studies outlined in this Proposal will provide insights into how receptors function as "on/off" switches in cells, thus aiding rational drug design for the development of new therapeutic agents.